Due to the slow response of nematic liquid crystals (typically 25 milliseconds (ms) to 100 ms), applications for nematic liquid crystals today have been mainly limited to those with relatively low data rates. This shortcoming is caused by the fact that the switching speed of nematic liquid crystals is often limited by its long relaxation time. Unlike the switching-on process, which is electric-field driven, the relaxation process (i.e. switching-off) is not electric-field driven and is generally a much slower process. This relaxation process is slower in a thicker cell since the restoring force provided by the alignment layers (substrate surfaces) to the bulk is weaker in a thick cell. In order for liquid crystal devices to switch at high speed, i.e. with short relaxation time, one often needs to use very thin cell gap (e.g. <2 μm) such that the restoring force provided by the alignment layers remains very high in the bulk of the liquid crystal. This thin cell gap however increases fabrication difficulty, especially when the panel size is large, and also limits the potential optical retardation since cell gap is limited. It is therefore anticipated that if a nematic liquid crystal mode capable of achieving very fast response (e.g. <5 ms) without the use of thin cell gap can be realized, it can greatly advance the development of high-speed applications in various areas such as those in optical communication systems (e.g., optical phase modulators) and advanced display systems, e.g., color sequential displays and television (TV) displays. Although a liquid crystal mode, known as Pi-cell Bend mode or Optically Compensated Bend (OCB) mode, capable of generating very fast response without the use of thin cell gap has been reported, this liquid crystal mode is unstable and requires a constant bias voltage.
With regard to Fringing-Field-Switching mode (FFS), the electrode design in the present invention is very similar to the FFS mode, as shown in FIG. 6, which was proposed by researchers in Korea for obtaining high efficiency wide-viewing-angle liquid crystal displays. The FFS mode developed in Korea has Parallel-Alignment (PA) at the off-state which leads to much slower response since the LC molecules in the whole bulk layer also switch without the restriction of the boundary effects, and the restoring force is mainly governed by the elastic constant K22 which is small. See “Fringing-Field-Switching mode (FFS)”, Seung Hong et al, Japanese Journal of Applied Physics, Vol. 39, p. L527, 2000.
Vertical-Alignment-In-Plane-Switching (VA-IPS) is discussed by W. Liu et al, in SID Digest '98, p. 319, 1998. Researchers proposed the VA-IPS mode which combined VA alignment with IPS electrode design. The VA-IPS mode can achieve a wide-viewing angle property but the response time is not very fast. A major difference between VA-IPS and the present invention is that the gap l between the positive and negative electrode is much larger in VA-IPS mode as shown in FIG. 7 whereas l is almost zero in the present invention (separated by a passivation layer only). Therefore the electric field in the VA-IPS mode is much more spread out and less localized which results in large switching of LC bulk layer and hence small boundary effect and slower response time.
Another liquid crystal mode of interest is the Optically Compensated Bend mode (OCB) discussed in Japanese Journal of Applied Physics, Vol. 39, Part 1, No. 11, H. Nakamura et al, p. 6368-6375, (2000). OCB is a liquid crystal mode that can generate very fast response without the use of very thin cell gap or special driving conditions. At off-state, this mode has Parallel Alignment (PA) with opposite top and bottom pre-tilts as shown in FIG. 8. Under a certain bias voltage, the molecules in the bulk are switched to vertical. Modulation of electric field above this bias voltage causes switching of the molecules near the boundaries. The OCB liquid crystal mode has very thin active LC layers and very strong boundary effects due to K33 and K11 as shown in FIG. 8. The OCB mode is however rather unstable since it involves the Splay to Bend transition and it also requires a constant bias voltage.
The following patent references are related to the use of liquid crystal display devices, but have no mention to Fringing Field Switching mode for Vertically Aligned liquid crystals.
U.S. Pat. No. 5,128,786 to Yanagisawa describes a LC display device wherein the image elements are arranged like a matrix with discontinuous electrodes. U.S. Pat. No. 5,179,460 to Hinata et al. discloses an input structure for a LC display that prevents electrode peeling and eliminates bending stresses and cracks in the electrode. U.S. Pat. No. 5,377,027 to Jelley describes a light-emitting LC display wherein a matrix area separates pixels. U.S. Pat. No. 5,781,259 to Shinomiya et al. describes another LC display device with transparent substrates, transparent pixels and transparent electrodes, the transparent pixels are separated from polymer portions with a shielding layer of metal foil. U.S. Pat. No. 6,031,593 to Morikawa et al. describes a spacing layer for LC display using light shielding layer. Pixels are formed with flattened film in between to suppress disinclination. None of the references suggest a configuration that uses self-imposed boundary layers by vertically aligned liquid crystals. Commercial demand is very strong for fast response, stable and easily fabricated liquid crystal devices.
The present invention provides a novel liquid crystal mode capable of producing very fast response, due to fast relaxation time, without the use of thin cell gap is disclosed. This invention is very stable and has very short relaxation time even at low voltages.